1. Field of the Invention
The invention relates to a method for operating a resonance measurement system, especially a Coriolis mass flow meter, the resonance measurement system comprising at least one electrical actuating apparatus, at least one electromagnetic drive as a vibration generator and at least one vibrating element which interacts with a medium, the electrical actuating apparatus making available an electrical excitation signal for excitation of the electromagnetic drive and the electromagnetic drive exciting the vibrating element to vibration in at least one natural form.
2. Description of Related Art
Resonance measurement systems of the aforementioned type have been known for years, not only in the form of Coriolis mass flow meters, but also as density measuring instruments or level detectors according to the tuning fork principle, as quartz carriages and belt viscosimeters. These resonance measurement systems are connected to a process/process medium, the process and process medium and resonance measurement system mutually influencing one another.
Resonance measurement systems are treated below using the example of Coriolis mass flow meters; this is not to be understood as limiting. It is irrelevant whether they are Coriolis mass flow meters with one or several measuring tubes, with straight or bent measuring tubes. Here, quite generally, those systems in which information about the process variables (measured variables) to be determined are encoded in the natural frequencies and/or those systems in which working points are placed at the natural frequencies of the measurement system are called resonance measurement systems. What is stated below can be applied to all systems which fall under this definition. In Coriolis mass flow meters, the measuring tube corresponds to the vibrating element of the resonance measurement system; this special configuration of the vibrating element does not constitute a limitation for the teaching which can be applied in general to resonance measurement systems either.
Resonance measurement systems which are made as Coriolis mass flow meters are used mainly in industrial process measurement engineering, where mass flows must be determined with high precision. The manner of operation of Coriolis mass flow meters is based on at least one measuring tube through which a medium flows—the vibrating element—being excited to vibration by a vibration generator, this vibration generator being an electromagnetic drive. In this electromagnetic drive, conventionally, an electric current flows through a coil, the action of a force on the vibrating element being linked directly to the coil current. In Coriolis mass flow meters, the manner of operation is based on the mass-burdened medium reacting on the wall of the measuring tube as a result of the Coriolis inertial force which has been caused by two orthogonal movements, that of the flow and that of the measuring tube. This reaction of the medium on the measuring tube leads to a change of the measuring tube vibration compared to the vibration state of the measuring tube in the absence of flow through it. The mass flow rate through the measuring tube can be determined with high precision by detecting these particulars of the vibrations of the Coriolis measuring tube which has been exposed to flow through it.
The natural frequencies of the Coriolis mass flow meter or the resonant parts of the Coriolis mass flow meter, essentially therefore the natural frequencies of the measuring tube as the vibrating element, are of special importance, because the working points of the Coriolis mass flow meters are conventionally placed on natural frequencies of the measuring tube in order to be able to impress the necessary vibrations for the induction of the Coriolis forces with a minimum energy expenditure. The vibrations which are then executed by the measuring tube have a certain mode which is called the natural mode of the respective excitation. Another reason for the special importance of natural frequencies in Coriolis mass flow meters is the direct physical linkage between the natural frequency of the measuring tube which has been exposed to flow through it and the effectively deflected vibrating mass (measuring tube and mass of the medium in the measuring tube); the density of the medium can be determined via this relationship.
It is known from the prior art that, in order to excite the vibrating element by a controller, a harmonic base signal as the controller output signal is generated in the form of a sinusoidal voltage and this sinusoidal voltage triggers the electrical actuating apparatus, the electrical actuating apparatus being designed to make available a corresponding power at its output in order to be able to trigger the electromagnetic drive in a suitable manner and with sufficient power; the electrical actuating apparatus is thus essentially the power link between the controller and the electromagnetic drive of the resonance measurement system. Usually known Coriolis mass flow meters are also equipped with a vibration sensor, since in the vibration of the vibrating element which is interacting with a medium usually there is physical information of interest about the medium, for example, the flow rate, the density and the viscosity.
In resonance measurement systems in industrial practice, the available electric power is often limited for different reasons. One reason for this limitation can be, for example, that the resonance measurement system is designed for the type of protection “intrinsic safety”. This yields manipulated variable limitations which lead to limitations of the electrical excitation signal and thus to nonlinearities when approaching and holding predetermined working points.
The invention is based on the finding that the nonlinearities which are caused for example, by limitations of manipulated variables lead to unwanted multi-frequency excitations of the resonance measurement system. For example, the load on the resonance measurement system when measuring multiphase flows or highly viscous materials is so great that limits in the drive chain and especially in the electrical actuating apparatus become active. In this way the resonance measurement system is excited not only at predetermined frequencies, but also at many unwanted frequencies. This changes the working point (vibration mode) and thus also the properties of the resonance measurement system such as the zero point and the sensitivity; it increases the measurement noise, reduces the accuracy of the evaluation of the measurement signals and increases the measurement uncertainty of the measured values.
Another problem with respect to the power consumption of the resonance measurement system, and thus, also to the level of the electrical excitation signals can be that the resonance measurement system is to be operated in different predetermined operating modes, in which certain modules have a large power demand so that “normal” measurement operation cannot be maintained for reasons of power technology. For example, the power demand in a diagnosis operation of the vibrating element can be so high that the driving power must be reduced for measurement operation.
To influence the power consumption, executing certain functions of the resonance measurement system only in sequence so that the instantaneous power demand does not exceed a predetermined quantity is known. For example, the driving of the measuring tubes of a Coriolis mass flow meter can be discontinued when sending the measurement data; this is important for example, in two-lead resonance measurement systems.
In many resonance measurement systems which are known from the prior art, the power limitation, and thus, also the manipulated variable limitation are simply ignored. But, this procedure leads to undefined states of the resonance measurement system, and thus, to major measurement uncertainties. An undefined state is present, for example, if the vibrating element is also excited with signals of unknown frequency in unintended natural modes in addition to known and intended excitations. As a result, the predetermined working point becomes uncertain; for example, in a Coriolis mass flow meter, the intended defined change in the momentum of the flowing mass particles is not possible.
Uncertainties in the working point then also cause model uncertainties in the evaluation of the response signals of the vibrating element, and thus, also further measurement uncertainties in the measurement results.